Active LED module with LED and transistor formed on same substrate

ABSTRACT

An LED module is disclosed containing an integrated driver transistor (e.g, a MOSFET) in series with an LED. In one embodiment, LED layers are grown over a substrate. The transistor regions are formed over the same substrate. After the LED layers, such as GaN layers, are grown to form the LED portion, a central area of the LED is etched away to expose a semiconductor surface in which the transistor regions are formed. A conductor connects the transistor in series with the LED. Another node of the transistor is electrically coupled to an electrode on the bottom surface of the substrate. In one embodiment, an anode of the LED is connected to one terminal of the module, one current carrying node of the transistor is connected to a second terminal of the module, and the control terminal of the transistor is connected to a third terminal of the module.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 14/204,965, filed onMar. 11, 2014, which is a continuation-in-part of application Ser. No.13/737,672, filed on Jan. 9, 2013, and also claims priority toprovisional application Ser. No. 61/788,967, filed on Mar. 15, 2013. Allapplications are assigned to the present assignee and incorporatedherein by reference.

FIELD OF THE INVENTION

This invention relates to light emitting diodes (LEDs) and, inparticular, to a single die containing driver circuitry in series withan LED to control current through the LED.

BACKGROUND

LEDs are typically formed as dies having an anode terminal and a cathodeterminal. An LED die is typically mounted on a larger substrate for heatdissipation and packaging. The substrate may contain additionalcircuitry, such as a passive electrostatic discharge device. The LED dieand optional substrate are then typically packaged, where the packagehas robust anode and cathode leads for being soldered to a printedcircuit board (PCB).

LEDs may be controlled by a current source to achieve a desiredbrightness. The current source may be a MOSFET or a bipolar transistorformed in a separate die. The current source and LED are typicallyconnected together by wires or a PCB.

Providing the current source separate from the LED die requires extraspace and interconnections, adding cost. Other disadvantages exist,including the possibility of mismatching components. It would bedesirable to provide a very compact LED module with an integratedcurrent source driver circuit.

Additional problems arise when driving multi-colored LEDs, such as in acolor display or for creating a white light source. An LED is a twoterminal electrical device with non-linear voltage versus currentcharacteristics. Below a particular voltage threshold, the LED is highimpedance. Above the threshold, the LED's impedance is much lower. Thisthreshold depends primarily on the bandgap of the semiconductor LED. Thebandgap is selected for a particular peak emission wavelength. Red LEDshave bandgaps on the order of 2 eV, blue LEDs have bandgaps on the orderof 3 eV, and green LEDs have bandgaps between 2 eV-3 eV. Since theforward voltage is directly related to the bandgap energy, red, green,and blue LEDs cannot simply be connected in parallel to output a desiredcolor or light; each color LED must have its own driver circuit. Thedifferent materials (e.g., GaAs, GaN, etc.) used to form the differentcolor LEDs also affect the forward voltages. Further, even within LEDsoutputting the same wavelength, their forward voltages vary due toprocess variations, so even connecting the same color LEDs in parallelis problematic. Providing a separate driver circuit for each LED andinterconnecting it to the LED adds space and cost. This added size isparticularly undesired when trying to minimize the size of an RGB pixelin a display.

LEDs can be organized in passive matrix addressable arrays. Forinstance, a set of LEDs can be connected with their cathodes connectedto a row select driver and their anodes connected to a column data bus.Several of these rows can be used to form a larger array addressable byrow and column. Providing a controlled current through an addressedrow-column will energize the LED(s) at the addressed location(s) to emitthe desired color and intensity of light, such as for a color pixel in adisplay. Since the interconnection between the LEDs has a non-zeroimpedance, the voltage drop throughout the interconnect network caninadvertently forward bias a non-addressed set of LEDs. Such incidentalforward bias will cause excess light in non-addressed segments, whichreduces light-to-dark contrast of the array.

It would be desirable to create integrated LED modules that avoid theabove-mentioned problems when connected in an addressable array.

It would also be desirable to create integrated LED modules where LEDsof different colors can be connected in parallel to form a high densityof compact RGB pixels.

It would also be desirable to create integrated LED modules of differentcolors that can be inexpensively packaged together in a single panel forgenerating light for backlighting, for general illumination, or for acolor display.

It would also be desirable to create an interconnection and addressingscheme for multiple LED modules to form a compact light or displaypanel.

SUMMARY

Problems related to parallel and addressable connections of LEDs, suchas in a color display, can be resolved by using active LED modules. Inone embodiment, a single vertical LED module includes an LED in serieswith a drive transistor (a voltage-to-current converter). Threeterminals are provided on the module: a positive voltage terminal, anegative voltage terminal, and a control terminal for controlling thecurrent through the LED. The difference between the voltages applied tothe positive and negative voltage terminals must be sufficient toenergize the LED to its full desired brightness when the controlterminal is supplied a maximum value control signal.

The control terminal may be connected to the gate or source of a MOSFETconnected in series with the LED. The control terminal is added so thatthe threshold non-linearity of the LED impedance is actively, ratherthan passively, controlled. For an LED module where voltage is providedacross the power terminals of the module, the low impedance state (wherethe LED is emitting light) is determined by the control voltage appliedto the control terminal. Such an active LED in a parallel or addressablenetwork of LEDs would always be in a high impedance state until thecontrol signal activates the low impedance state. This active impedancecontrol reduces sensitivity to forward voltage and parasitic voltagedrops and reverse current paths.

In one example, red, green, and blue LED modules are connected inparallel in an array for a multi-color display, where any set of RGBLEDs (forming a single pixel) is addressable by applying the samevoltage across the voltage terminals of the three modules. The controlterminal of each module is connected to a different variable controlvoltage to achieve the desired brightnesses of the red, green, and blueLEDs in the pixel. The control voltages are applied in sequence at 60 Hzor greater so that the different forward voltages of the RGB LEDs are nolonger relevant.

In another embodiment, modules are connected in series and parallel fora white light source, where the white point is set by the relativecombination of red, green, and blue light. The control voltage for eachcolor and the duty cycle for each color are set to achieve the desiredwhite point.

In other embodiments, various circuits are integrated with the LED tomake the brightness of the LED less sensitive to variations in inputvoltage.

The modules are extremely compact since the footprint may beapproximately the same as a single conventional LED die (e.g., 0.5 mm²−1mm²). If the modules are printable, the footprint is much smaller.

In one embodiment, the LED layers and transistor layers/regions areformed on the same surface of the substrate. In one example, the bottomsurface of the substrate is a cathode electrode. N-type and p-typelayers are epitaxially grown over the substrate to form the LED. TheseLED layers may be GaN-based. The p-type layer for the LED is connectedto an anode electrode. A center area of the LED layers is etched away,and a p-channel MOSFET (or other type of transistor) is formed in theexposed surface. The control terminal of the MOSFET is the thirdterminal of the module.

When the MOSFET is turned on, current flows vertically through thesubstrate, then laterally through the MOSFET, then vertically throughthe LED to turn on the LED. This one-side-processing technique may beused form transistors, such as MOSFETs or bipolar transistors, havingeither polarity.

Blue or green LEDs may be grown on a SiC layer or a GaN layer. An SiCsubstrate is conductive, so the substrate itself can conduct thevertical current. For a GaN layer grown on a sapphire substrate, thesubstrate is removed, such as by laser lift-off or grinding.Alternatively, a conductive via is formed through the sapphiresubstrate. In another embodiment, a silicon substrate is provided andintermediate layers are grown as buffer layers between the Si and theGaN layers to transition between the two lattice constants. If a siliconsubstrate is used, the LED is formed in the GaN layer, and the drivertransistor is formed in the Si. Either device may be formed first. GaAssubstrates may be used for forming red LEDs.

To avoid having to conduct current vertically through any of thepossible substrates, a through-via in the substrate may be filled with aconductive material.

In one embodiment, the resulting LED modules are made very small and arescreen printed on a display panel or printed using flexography.Printable modules may have a top surface area range of between, forexample, 50-10,000 um². An array of small groups of the modules may beprinted, where the modules in each group are connected in parallel toform a single color pixel having a desired maximum brightness. In oneembodiment, the packaging for the module is also formed by printing.

In a large lighting system using hundreds of medium power LEDs, it wouldbe impractical to provide a conventional drive circuit for each of theLEDs. For such white light sources, many LEDs are typically connected inseries, and a high voltage is connected across the string. In the priorart, providing such a high voltage sometimes requires a step upregulator, adding cost to the system. The present invention inherentlyprovides each LED with its own driver, allowing many LEDs, even ofdifferent colors, to be connected in parallel so that they may be drivenwith a low voltage (e.g., 5 volts). Providing each LED with its owndriver also enables each LED to be controlled to output a desiredbrightness despite process variations, changes in brightness withtemperature, and changes in brightness with age.

Various module embodiments are described along with various addressablearrays of LED modules that are suitable for LED displays or white lightsources.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a simplified cross-sectional view of a single singulatedLED/driver module in accordance with one embodiment of the invention.

FIG. 2 illustrates a PMOS driver transistor connected to the anode of anLED.

FIG. 3 illustrates a pnp bipolar driver transistor connected to theanode of an LED.

FIG. 4 illustrates an NMOS driver transistor connected to the anode ofan LED.

FIG. 5 illustrates an npn bipolar driver transistor connected to theanode of an LED.

FIG. 6 illustrates a PMOS driver transistor connected to the cathode ofan LED.

FIG. 7 illustrates a pnp bipolar driver transistor connected to thecathode of an LED.

FIG. 8 illustrates an NMOS driver transistor connected to the cathode ofan LED.

FIG. 9 illustrates an npn bipolar driver transistor connected to thecathode of an LED.

FIG. 10 illustrates various ways to apply fixed voltages and variablecontrol voltages to the three terminals of the module in FIG. 1,depending on the position of the LED and the type of driver transistorused.

FIG. 11 illustrates a singulated module die after packaging, such as ina panel, where conductor layers contact the three terminals of themodule.

FIG. 12 is a cross-sectional view of one type of MOS transistor-LEDconfiguration, where the structure may be symmetrical around a centerline CL. In an actual device, the LED portion would be much wider thanthe transistor portion. The light may exit upward or downward.

FIG. 13 is a top down view of a singulated hexagonal module, where thedriver transistor is surrounded by the LED.

FIG. 14 is a top down view of a singulated rectangular module, where thedriver transistor is surrounded by the LED.

FIG. 15 is a cross-sectional view of one type of bipolar transistor-LEDconfiguration, where the structure may be symmetrical around a centerline.

FIG. 16 is a cross-sectional view of another type of bipolartransistor-LED configuration, where the structure may be symmetricalaround a center line.

FIGS. 17-21 are cross-sectional views of other types of MOStransistor-LED configurations, where the structures may be symmetricalaround a center line.

FIG. 22 is a flowchart of process steps used to form an LED/drivermodule where the LED portion is formed before the transistor portion.

FIG. 23 is a flowchart of process steps used to form an LED/drivermodule where the transistor portion is formed before the LED portion.

FIG. 24 illustrates RGB LED modules connected in parallel for a colordisplay or for generating white light.

FIG. 25 illustrates how the RGB LEDs in FIG. 24 may be sequenced usingthe control voltage to create any color, including white light.

FIG. 26 illustrates separate RGB LED modules packaged together, such asin a color display.

FIG. 27 illustrates how transistors and other circuitry may beintegrated in the same substrate as the LED to form voltage clamps,current regulators, or other circuits. No external control voltage isrequired. This results in 2-terminal LED modules, such as RGB modules,where the modules are connected in parallel for a color pixel, includinga white light pixel.

Elements that are the same or similar in the figures are labeled withthe same numeral.

DETAILED DESCRIPTION

FIG. 1 is a cross-sectional view of a singulated LED module 10. In oneembodiment, the size (footprint) of the module 10 is about 0.05 mm²-1mm². If the modules 10 are to be printed as an ink, the sizes may besmaller. In the examples, the driver transistor is formed in the centerarea, and the LED is formed surrounding the transistor.

If the light is to exit the top surface, as light rays 12, the topterminals T2 are made small so as to block a minimum of light, and areflector may be formed on the bottom surface of the substrate. If thesubstrate is opaque to the LED light, such as silicon, the light wouldexit the top surface, and no bottom reflector is needed. Depending onthe application and the substrate, the light may even exit the bottom ofthe substrate.

As described below, the substrate may be any type of substrate thatenables the growth of the LED epitaxial layers and enables the drivertransistor portion to also be formed over/in the same substrate. In someexamples, the LED is GaN-based and emits blue light or green light. Thelight may be converted by a phosphor layer. The LEDs may also be GaAsbased and emit longer wavelengths, such as from green to red. Thestarting substrates may be Si, SiC, sapphire, GaN, or other suitablesubstrate.

In one embodiment of the module 10, schematically shown in FIG. 6, alow-side PMOS transistor is the driver transistor. To control the module10 of FIG. 1 to emit light, a positive voltage is applied to theterminal T2 (connected to the anode of the LED), a negative voltage isapplied to the bottom terminal T1 (connected to the drain of the PMOStransistor), and a gate-source voltage exceeding the PMOS transistor'sthreshold (i.e., sufficiently more negative than the voltage applied toterminal T2) is applied to the terminal T3. The source is connected tothe cathode of the LED internal to the module 10 to create a seriesconnection between the driver transistor and the LED. In one embodiment,to forward bias the LED, the voltage differential across terminals T3and T1 is greater than 2 volts. For a blue LED, the required voltagedifferential may be greater than 4 volts.

The driver transistor may be any type of transistor and may be ahigh-side transistor, a low-side transistor, a PMOS transistor, an NMOStransistor, an npn bipolar transistor, or a pnp bipolar transistor.FIGS. 2-9 illustrate various possible configurations and types of thedriver transistor and the LED.

FIG. 2 illustrates a PMOS driver transistor connected to the anode of anLED.

FIG. 3 illustrates a pnp bipolar driver transistor connected to theanode of an LED.

FIG. 4 illustrates an NMOS driver transistor connected to the anode ofan LED.

FIG. 5 illustrates an npn bipolar driver transistor connected to theanode of an LED.

FIG. 6 illustrates a PMOS driver transistor connected to the cathode ofan LED.

FIG. 7 illustrates a pnp bipolar driver transistor connected to thecathode of an LED.

FIG. 8 illustrates an NMOS driver transistor connected to the cathode ofan LED.

FIG. 9 illustrates an npn bipolar driver transistor connected to thecathode of an LED.

The transistors may also be HEMTs, MESFETs, or other types.

FIG. 10 identifies various ways to control an LED module, depending onthe position of the LED and the type of transistor used. For example,instead of controlling a MOSFET by controlling its gate voltage, thegate voltage may be fixed (e.g., positive) and the source voltage may becontrolled to achieve the desired Vgs.

FIG. 11 is a cross-sectional view of the module 10 packaged toencapsulate it and to provide conductors for applying power and controlsignals to the module 10. The encapsulated module 10 may form part of adisplay panel in which many modules are encapsulated in the same panel.In FIG. 11, a substrate 14 is provided, such as a transparent plastic orglass panel, with a conductor 16 for direct bonding to the terminal T1of the LED module 10. In a panel, there may be many conductors 16connected to various LED modules in an array, or a single conductorsheet may connect the LED modules in parallel. The metal conductor 16 isultimately connected to a power terminal.

The modules 10 may be positioned by an automatic pick and place machine,or the modules 10 may be printed as an ink. If the modules 10 areprinted as an ink, each module 10 may be microscopic (e.g., less than200 microns across), and multiple modules for being controlled in thesame way can be printed as a small group (e.g., a pixel), and themodules in the group are connected in parallel to generate the desiredamount of light. The microscopic modules in each group will be generallyrandomly located in a small pixel area. The printing and patterning maybe by flexographic printing, or screen printing, or other types ofprinting. When the ink is cured, such as by heating, the solventevaporates, and the bottom terminals T1 of the modules 10 becomeohmically connected to the underlying conductor 16. The shape of themodules 10 causes the modules 10 to be oriented correctly duringprinting.

Light from the LED may be emitted downward through the module 10 andsubstrate 14 or upward. If the light is to exit from the bottom of thesubstrate 14, the conductor 16 and substrate 14 would be transparent. Ifthe light is to exit upward and the module 10 is transparent, the module10 may include a reflective layer 20 as the terminal T1 electrode.

A dielectric layer 18 is then printed over the substrate 14 toencapsulate the sides of the module 10. The dielectric layer 18 may alsoencapsulate other modules supported by the substrate 14.

The module 10 may have a reflective film 20A formed on its sides priorto encapsulation to prevent side light emission, or the dielectric layer18 may be reflective, such as white. Alternatively, side light from theLEDs is reflected upward and downward by the dielectric layer 18, suchas where the dielectric layer 18 contains white titanium oxideparticles. In such a case, the substrate 14 may be reflective so alllight ultimately exits through the top surface of the panel.

A second conductor 22 is formed over the dielectric 18 to contact theterminal T2. The conductor 22 may be transparent if light is to exit thetop surface. A dielectric layer 24 is formed over the conductor 22, anda third conductor 26 is formed over the dielectric layer 24 to contactthe terminal T3. The conductor 26 may be transparent. In one embodiment,the conductors 16, 22, and 26 are narrow column and row lines of anaddressable LED panel, such as a color display or a white light source.All the conductors may be printed.

A display panel may include many thousands of LED modules 10 of variouscolors, such as the primary colors red, green, and blue, or othercolors, such as yellow and white. All LEDs may be blue LEDs, with thered and green colors being formed by red and green phosphors. If thepanel is a white light panel to be used for general illumination or as abacklight for an LCD, each LED may be a blue LED coated with a phosphorthat adds green and red components to form white light. The panel may beon the order of 2 mm thick and be any size. The various LEDs may beconnected in any configuration, such as series, parallel, or acombination to achieve the desired voltage drop and current.

FIG. 12 illustrates a portion of the internal structure of a singlemodule 10 having a center transistor portion 30 and an outer LED portion32 formed over the top surface of the substrate 34. The structure may besymmetrical about the center line CL so the all portions of the LEDsurrounding the transistor are driven with an equal current. The circuitschematic of FIG. 12 is shown in FIG. 6, where a high-side LED isconnected in series with a low-side PMOS transistor.

As shown in FIGS. 13 and 14, the LED portion 32 may surround thetransistor portion 30 to maximize the light generated. The modules 10may be hexagonal or rectangular (including a square).

The gate terminal of the transistor is shown connected to terminal T3,the LED's anode is shown connected to terminal T2, and the bottom of thegrowth substrate 34 is connected to terminal T1. If the substrate 34 isnot sufficiently conductive to conduct the vertical current, a throughvia 36 may be laser-drilled or etched and then filled with a conductivematerial 38. The walls of the via 36 may be first coated with a thindielectric layer if needed. A wrap-around conductor may be used insteadof a via to conduct the vertical current.

In one embodiment, the substrate 34 is silicon, GaN, SiC, GaAs or othersuitable material. If a sapphire substrate is used as the growthsubstrate, for growing GaN layers, the sapphire may be removed by laserlift-off or grinding. Therefore, the substrate 34 will be the remainingGaN layers. If a silicon growth substrate is used, intermediate bufferlayers are epitaxially grown over the silicon surface to provide latticematching to the ultimate GaN layers in which the blue or green LED isformed. For red LEDs, the substrate 34 may be GaAs.

If the substrate 34 is conductive, an insulator layer or highresistivity layer 40 is grown or doped to effectively insulate the LEDportion 32 from the transistor portion 30. The high resistivity layer 40may be an undoped or counter-doped layer.

Over the high resistivity layer 40 is grown an n-type layer 42, such asa GaN-based layer. Various other layers (not shown) may be formed overthe n-type layer 42 to form an active layer of a heterojunction LED,using conventional techniques. A p+ type layer 44 (also a GaN-basedlayer) is then formed to complete the LED layers.

Next, portions of the p+ type layer 44 and n-type layer 42 are etchedusing a conventional photolithographic masking and etching process toexpose the central area of the n-type layer 42 and to form an isolationtrench 45 around the transistor portion 30.

A thin gate dielectric layer 46 is formed, followed by depositing thegate layer, such as metal or polysilicon. The gate layer and dielectriclayer 46 are then etched to form the gate 48.

Standard masking and dopant implantation techniques are then used toform the p+ type source region 50 and p+ type drain region 52 in then-type layer 42 self-aligned to the gate 48.

A dielectric layer 54 is deposited and etched to expose thesemiconductor areas that are to be contacted by a metal layer. The metallayer is then deposited and patterned to form the various metal contactsand connections. The metal portion 56 forms an anode contact for theLED, the metal portion 58 connects the source of the PMOS transistor tothe cathode of the LED, and the metal portion 60 connects the drain ofthe PMOS transistor to the conductive material 38 in the via 36. Themetal portion 58 may also short the p+ type region (source) to then-type layer 42 (body region). A backside metal layer 62 connects to theconductive material 38 in the via 36. If the light is to exit throughthe bottom of the substrate 34, the backside conductor may betransparent or narrow traces of an opaque metal so as not to block asubstantial amount of light.

Depending on the materials used, the transistor may be formed in Si,GaN, SiC, GaAs or other material.

Current flows vertically through the conductive material 38 in the via36, laterally through the transistor, and vertically through the LED toturn on the LED. The light exits the top of the module 10. In an actualembodiment, the LED portion 32 may extend out more, and the anode metalportion 56 may only contact the edge of the LED so as not to block toomuch light. A transparent conductor or narrow metal lines may bedeposited over the p+ type layer 44 to help spread current.

If the LED layers are GaN-based (hereinafter GaN), and if the substrateis not GaN, such as silicon, the transistor may be directly formed inthe substrate (or a doped top layer) after etching away the LED GaNlayers. FIGS. 22 and 23, described later, generally describe thedifferences in forming the module when the LED layers are formed priorto the formation of the transistor and when the LED layers are formedafter the formation of the transistor.

FIGS. 15-21 illustrate other module designs.

FIG. 15 illustrates an LED/driver structure schematically illustrated inFIG. 9. An n+ type conductive substrate 64 has an n-type layer 65 grownover it or doped by implantation. An insulating layer 66 (e.g., anundoped layer or other high resistivity layer), is formed over then-type layer 65, followed by growing the n-type and p+ type LED layers42/44 (and any active layer). The LED layers and insulating layer 66near the center of the module are then etched away, and the p-type base66 and n+ type emitter 68 are implanted to form the npn bipolartransistor. The metal portions 56, 58, and 60 are deposited to form theinterconnections and contacts leading to the terminals T2 and T3. Thebackside metal layer 62 forms the T1 terminal.

FIG. 16 is similar to FIG. 15 but the substrate 70 is an n-type and notvery conductive. So a via 36 filled with conductive material 38 connectsthe metal portion 60 (base contact) to the backside metal layer 62.

FIG. 17 is similar to FIG. 12 except the substrate 72 is a highconductivity n+ type, so no via is required for conducting the verticalcurrent to the backside metal layer 62. Also, the transistor regions areformed in the substrate rather than in any n-type LED layer. Thetransistor's p+ type regions 50/52 are implanted in an n-type layer 74grown over the n+ type substrate 72. An n+ type sinker 76 and the metalportion 60 electrically couple the drain current to the n+ typesubstrate 72.

FIG. 18 is similar to FIG. 17 but uses a p+ type substrate 78. Thetransistor's p+ type regions 50 and 80 are formed in the n-type layer74. The p+ type region 80 is deep to make contact to the p+ typesubstrate 78 to conduct the vertical current. An n+ type region 82 andthe metal portion 60 short the p+ type region 80 to the n-type layer 74.

FIG. 19 is similar to FIG. 17 but the substrate 86 is an n-type, so thevertical current is conducted by the conductive material 38 in the via36. The p+ type region 52 is shorted to the substrate 86 by the n+ typeregion 88 and the metal portion 60.

FIG. 20 illustrates the transistor as an n-channel DMOS transistor. Thesubstrate 90 is p+ type, and an n-type layer 74 is grown over thesubstrate 90. The p-type body 92 is implanted in the n-type layer 74,and a deep p+ type region 94 connects the body 92 to the p+ typesubstrate 90. N+ type regions 96 and 98 are formed, where a sufficientlypositive gate bias inverts the channel in the body 92 to conduct currentlaterally between the n+ type regions 96/98. The current is conductedvertically by the metal portion 60, the p+ type region 94, and thesubstrate 90. FIG. 8 best represents schematically the circuit of FIG.20, where the NMOS transistor of FIG. 8 is the n-channel DMOS transistorin FIG. 20.

FIG. 21 is schematically illustrated by FIG. 2, where the LED is alow-side LED and the PMOS transistor is a high-side transistor. Some ofthe layers are similar to those in FIG. 17 and have been similarlynumbered. The main differences are that the LED's cathode electrode 102is connected to the terminal T2, and the metal portion 58 connects thep+ type region 50 (the drain of the PMOS transistor) to the p+ typelayer 44 (anode) of the LED. Thus, the bottom of the n+ type substrate72 serves as the anode of the module.

Many other related circuits can be fabricated to be equivalent to thevarious schematic circuits of FIGS. 2-9.

FIG. 22 is a flowchart of process steps used to form an LED/drivermodule where the LED portion is formed before the transistor portion.

In step 106, a starting substrate is provided. The substrate may be Si,SiC, GaN, GaAs, etc.

In step 108, assuming the LED is a blue LED, the LED's n-type, active,and p-type GaN-based layers are epitaxially grown over the substrate.Depending on the type of substrate, an insulator layer (e.g., undopedlayer) may be grown as an intermediate layer between the substrate andthe LED layers. The intermediate layers may also serve as latticematching layers.

In step 110, one or more of the LED layers are etched to form thetransistor area, such as in the center of the LED. Depending on thesubstrate material, the transistor may be formed in silicon, SiC, GaN,or GaAs. An FET transistor may be formed in any of those materials,including a JFET where a thin semiconductor region acts as the gate.

In step 112, the transistor's n and p-type regions are formed byimplantation. The gate, if any, is also formed.

In step 114, the metal contacts to the various layers/regions areformed.

In step 116, the wafer on which the modules are fabricated is passivatedand singulated to form the individual modules, such as the module 10 inFIG. 1. The modules may then be printed or placed on another substrateand packaged for connecting the package leads to the three terminals ofthe modules. The package may be an individual package or a packagecontaining a plurality of the same or different modules. For example, apackage may be a flat display panel containing modules emitting blue,green, and red light.

FIG. 23 is a flowchart of process steps used to form an LED/drivermodule where the transistor portion is formed before the LED portion.

In step 118, a starting substrate is provided, with or without anydifferently-doped top layer in which the transistor will be formed.

In step 120, assuming the transistor can be formed in the top surface ofthe substrate, the various transistor regions are formed along with anygate.

In step 122, the transistor area is masked. The LED portion may surroundthe transistor portion.

In step 124, the LED's GaN layers are epitaxially grown over thesubstrate in the exposed areas.

In step 126, the metal contacts to the various layers/regions areformed.

In step 128, the wafer on which the modules are fabricated is passivatedand singulated to form the individual modules, such as module 10 inFIG. 1. The modules may then be printed or placed on another substrateand packaged for connecting the package leads to the three terminals ofthe modules.

By forming the driver transistor in the same substrate as the LED, thereis no extra material cost for the driver and no significant real estatetaken up by the driver. Therefore, the module may serve as a tiny singlecolor pixel in a display. Since there is no distance between the LED andthe driver, there is no parasitic capacitance that could delay theenergization current to the LED. Therefore, the display pixels may bedriven at a higher speed than conventional LED displays, where thedriving current is provided remotely.

FIG. 24 illustrates circuitry in a single package containing at leastthree LED modules. The package may be a display panel containing anarray of addressable LEDs. One module includes an LED 130 that emits redlight, one module includes an LED 132 that emits green light, and onemodule includes an LED 134 that emits blue light. The LEDs 130 and 132may be phosphor coated blue LEDs. The modules include p-channel MOSFETs136, 137, and 138, similar to FIG. 2. The package includes conductors140 (e.g., X-address lines) that electrically connect the sourcestogether and conductors 142 (other X-address lines) that connect theLED's cathodes together so that the modules are connected in parallel.Each LED is controlled by a separate control voltage applied to the gateof its respective MOSFET by conductors 144-146 (e.g., Y-address lines).In this way, any color light, including white, may be generated by thepackage. The three modules may form a single color pixel in a display ormay be part of a white light panel.

The advantage of the integrated modules, when controlling differentcolor LEDs connected in parallel, is that the modules can have twocommon terminals connected to the positive and negative voltages, withthe third terminal selecting a single LED at a time. By only turning onone color LED at a time, its forward voltage does not affect the voltageacross the other LEDs. For example, if the control voltages were allpulled low concurrently, the low forward voltage of the red LED 130would prevent the green and blue LEDs from turning on. As long as onlyone LED color is active at a time, then there is no conflict betweendifferent forward voltages. The turn-on duration of the different LEDcolors can be divided in time (time division multiplexing), and thecontrol voltage level can be adjusted for the active LED forwardvoltage. In one embodiment, the control voltages applied to the gates ofthe MOSFETs 136-138 are provided sequentially at a frequency above about60 Hz, where the relative duty cycles of the control voltages controlthe perceived color of light.

FIG. 25 is an example of the relative on-times of the red, green, andblue LEDs 130, 132, 134 in a single cycle for controlling the lightemission from the three modules. The control voltages may be differentfor each color LED to cause the respective LED to emit a certainpredetermined flux level (e.g., a nominal maximum brightness), wherebyany overall brightness level and color, including white or neutrallight, can be achieved by controlling the absolute on-times (forbrightness) and the relative on-times (for color) per cycle.

FIG. 26 illustrates a package 150 containing three LED modules 152-154.The package may be an entire panel of addressable LEDs, and FIG. 26 mayjust illustrate a small portion of the panel. Module 152 contains a redLED, module 153 contains a green LED, and module 154 contains a blueLED. In the example of FIG. 26, the cathode terminals T1 of the LEDs areconnected together by the conductor 156, supported by the substrate 158.The direction of light emission from the package 150 may be up or down.The various conductor layers may be opaque, reflective, or transparent,depending on which direction the light exits. The transistors in themodules 152-154 are p-channel MOSFETs, where a gate voltage sufficientlybelow the source voltage turns on the transistor and LED. The gates ofthe transistors are connected in common by the conductor 160, and thesources of the transistors are separately contacted by conductors162-164, extending into and out of the drawing page. The voltage acrossthe conductors 156 and 160 is higher than the forward voltage of any ofthe LEDs. By individually controlling the source voltages in atime-division fashion, the respective transistors can be separatelycontrolled to conduct any current to control the mix of the RGB colors.

The dielectric layers 18 and 24 may be the same as in FIG. 11.

Alternatively, the sources of the transistors in FIG. 26 may beconnected together by a conductor replacing conductors 162-164, and thegates are separately contacted by conductors replacing the commonconductor 160 to allow individually controlling the transistors via thegate voltage.

In one embodiment, the structure of FIG. 26 represents a single 3-modulepackage with five terminals. In another embodiment, the structure ofFIG. 26 is only a portion of a much larger panel having a singlesubstrate 158, where each color pixel location contains the three RGBmodules. The dielectric 18 may be a single dielectric layerencapsulating all the modules on the panel. The pixels in a row may beaddressed by applying a voltage across row (X) conductors 156 and 160,and the individual LEDs at any pixel location in an addressed row may beturned on by applying a suitable control voltage to the column (Y)conductors 162-164. Many modules in a column may receive the samecontrol voltage, but LEDs in a non-addressed row will not turn on.

In high power (>0.1 W/in²) lighting applications (including backlightingan LCD) where many LEDs can be on at the same time, it is advantageous,for a given power, to increase the operating voltage and reduce thecurrent. Power losses in the printed interconnects are proportional tothe square of the current; therefore efficiency can be increased byconnecting multiple LED modules in series (such as modules in a singlecolumn), which sum to a larger voltage but lower current. Accordingly,the connectors between modules may connect modules in a combination ofseries and parallel.

If the panel of FIG. 26 is to be used for general lighting, there is noneed for row addressing, and the columns of series red, green, and blueLEDs are just addressed in a rapid time division repeating pattern byapplying control voltages to the control terminals. To the human eye,the colors blend together without flicker. Either the on-time per color,the particular number of LEDs in a series, or the control voltage percolor may be selected to generate the desired perceived color (e.g.,white point). The emitted color may be controlled to be selectable bythe user.

For a lighting panel (as opposed to a color display with addressablepixels), convergence of the individual RGB elements is important toreduce visual nuisances of unmixed color. Therefore it is desirable topattern the individual LEDs colors in a regular pattern that willconverge into the desired color within a particular diffusion length.Secondly, for warm white colors, considerably more red power is neededthan green and blue. An RGB array having a regular pattern and twice asmany red LEDs as green and blue LEDs may be used.

Within a single module, diodes, resistors, and transistors may beformed. The base or gate of the transistor may be internally connectedto a resistor to form a voltage or current limiter, or other circuit.Therefore, the modules may only need two operating voltage terminals andno control terminal. This may be suitable for general lighting purposesor backlighting purposes. The drivers are generally characterized as avoltage-to-current (V-to-I) driver.

FIG. 27 illustrates 2-terminal modules 170, 171, and 172 connected inparallel, where the three modules 170-172 contain red, green, and blueLEDs to form a single light element in a light panel, such as forgeneral illumination or backlighting. The circuitry is set for eachcolor LED to emit the desired brightness (by setting a certain currentthrough the LED) while also setting the desired voltage drop across themodule to allow each of RGB LEDs to turn on. The integrated LED modulescan be paralleled to achieve uniform luminance without other externalcomponents. In another embodiment, all the LED are the same color,including blue LEDs with a phosphor coating to generate white light.

The integration of the driver and LED into a single integrated circuitchip reduces intrinsic and parasitic uncertainty of the LED and theinterconnection to the global system. The integration also greatlyreduces the size and cost of the circuit compared to usingnon-integrated V-to-I drivers.

Additionally, providing each LED with its own controllable driverenables each LED to be controlled to output a desired brightness despiteprocess variations, changes in brightness with temperature, and changesin brightness with age.

The preceding examples have mostly used MOSFETs and bipolar transistors;however, the scope of this invention is not limited by the transistortechnology. Realizations can be created using a CMOS, BiCMOS, BCD, DMOSor other integrated circuit processes. Additional transistortechnologies not shown could be used as well such as JFET, IGBT,Thyristor (SCR), Triac, and others.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore, the appended claims areto encompass within their scope all such changes and modifications asfall within the true spirit and scope of this invention.

What is claimed is:
 1. A lighting device comprising: a first growthlayer having a top surface and a bottom surface; a light emitting diode(LED) having at least a first LED layer of a first conductivity type anda second LED layer of a second conductivity type, the first LED layerand the second LED layer being epitaxially grown over the top surface ofthe first growth layer, wherein the LED is formed to have a centralopening; and a first transistor formed over or in the top surface of thefirst growth layer, the first transistor comprising a first currentcarrying node, a second current carrying node, and a control node,wherein the first transistor is at least partially formed in the openingin the LED such that the LED surrounds the first transistor and emitslight in an area surrounding the first transistor when the firsttransistor is turned on, wherein the first current carrying node iselectrically coupled to the first LED layer with a metal layer formedover the first growth layer, and wherein the second current carryingnode is electrically coupled to a bottom electrode formed on the bottomsurface of the first growth layer, such that the first transistor isconnected in series with the LED and the LED is turned on by asufficient forward current flowing between the bottom electrode and thesecond LED layer when the first transistor is turned on.
 2. The deviceof claim 1 wherein the first growth layer is a silicon substrate and theLED is GaN-based.
 3. The device of claim 2 further comprising at leastone buffer layer epitaxially grown over the silicon substrate to providelattice matching between the substrate and the LED layers.
 4. The deviceof claim 1 wherein the first transistor and the LED are both GaN-based.5. The device of claim 4 wherein the first growth layer comprises aGaN-based layer that was epitaxially grown over a growth substrate, andwherein the growth substrate has been removed.
 6. The device of claim 1wherein the first growth layer is conductive.
 7. The device of claim 1further comprising a through-via formed through the first growth layer,wherein the through-via is at least partially filled with a conductivematerial to provide a vertical conductive path between the bottomelectrode and the second current carrying node of the first transistor.8. The device of claim 1 further comprising an insulating layer over thefirst growth layer, wherein the LED is formed over the insulating layer,to electrically insulate the second carrying node of the firsttransistor from the LED when the first transistor is in its off state.9. The device of claim 8 further comprising the first LED layer grownover the insulating layer and a semiconductor layer of the firsttransistor also grown over the insulating layer.
 10. The device of claim8 further comprising the first LED layer grown over the insulating layerand a semiconductor region of the first transistor being a first dopedregion in the first growth layer.
 11. The device of claim 10 wherein thefirst transistor is a bipolar transistor, wherein the first growth layeris a collector region, the first doped region is a base region, thefirst transistor further comprising a second doped region in the baseregion forming an emitter.
 12. The device of claim 10 wherein the firsttransistor is an MOS transistor, wherein the first growth layercomprises a channel region, the first transistor further comprising asource region and a drain region on opposite sides of the channelregion.
 13. The device of claim 10 wherein the first transistor is aDMOS transistor, the first transistor further comprising a sourceregion, a drain region, and a body region, wherein the source region,the drain region, and the body region are doped regions in the firstgrowth layer.
 14. The device of claim 8 wherein the insulating layer isa doped top surface of the first growth layer.
 15. The device of claim 1wherein the second LED layer is grown over the first LED layer.
 16. Thedevice of claim 15 wherein the first current carrying node of the firsttransistor is connected to the first LED layer by the metal layer. 17.The device of claim 15 wherein the first current carrying node of thefirst transistor is connected to the second LED layer by the metallayer.
 18. The device of claim 1 wherein the first LED layer and thesecond LED layer are grown above the first growth layer, and dopedregions of the first transistor are formed in the first growth layer.19. The device of claim 1 wherein the first growth layer comprises afirst portion of a first conductivity type below a second portion of asecond conductivity type.